1. Field of the Invention
This invention relates to the recycling of rubber and other crosslinked polymeric materials, and in particular, to a continuous process for devulcanizing such materials so that the materials can be reused.
2. Description of the Prior Art
The disposal of used tires has become an increasing environmental concern. Waste piles and land fills filled with discarded tires have become common sights. Until recently, there has not been a satisfactory way of dealing with the disposal or reuse of tires and other products made of natural or synthetic rubber reinforced with other materials. While some tires have been used as retaining walls or traffic barrel anchors or have been used in other ways, the demand for such use is very limited, and it does not provide an adequate means for absorbing all of the tires which are discarded.
Various treatments have been explored in an attempt to recover the rubber from the tires in a form in which it can be reused, but these attempts have generally been unsuccessful or limited. One process, however, which has developed some interest is an ultrasonic devulcanization process in which the cured rubber from tires and other products, such as hoses and belts, is ground into particles and exposed to ultrasonic energy in a controlled process. When properly exposed to the ultrasonic energy, the carbon--sulfur and sulfur--sulfur bonds of the vulcanized rubber break, creating a material which is substantially devulcanized and can then be reused in the manufacture of rubber products.
Apparatus and methods for devulcanizing rubber using ultrasonic energy are described in detail in U.S. Pat. Nos. 5,258,413 and 5,284,625, issued to Isayev et al. While these apparatus and methods are generally effective, they do not provide for a method by which large quantities of material can be processed continuously in a cost effective manner. One of the problems encountered in designing a continuous ultrasonic devulcanization process is providing continuous exposure of the material to the ultrasonic energy in an efficient manner such that the material is suitably devulcanized. In the aforementioned U.S. Pat. Nos. 5,258,413 and 5,284,625, this was accomplished by positioning the ultrasonic horn in a coaxial direction within the flow of the material at the extruder exit. This required that all of the material flow past the ultrasonic horn since the horn essentially blocked the exit through which the material flowed. While this design was generally effective in devulcanizing the rubber, it limited the amount of material which could be processed, since the position of the horn severely restricted the flow of the material as it came from the extruder exit.